Stars are born near our galaxy’s black hole

Observations confirm theory that monster can be a stellar mother

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The artist's depiction demonstrates what scientists believe is happening very close to the Milky Way's central black hole. The supermassive black hole is surrounded by a disk of gas (yellow and red). Massive stars, shown in blue, have formed in this disk, while small disks represent where stars are still forming.

Black holes are best known for ripping stars apart, but new observations of the supermassive black hole at the center of the Milky Way show that it’s actually helping stars form.

Until now, scientists had disagreed about the origin of a collection of massive stars orbiting less than a light-year from our galaxy’s central black hole, which scientists call Sagittarius A*. The stars were first seen by infrared telescopes.

The new finding, based on observations from the Chandra X-ray Observatory, confirms the theory that black holes can help form massive stars and gives more support to the idea that black holes play a big role in galaxy formation.

“In one of the most inhospitable places in our galaxy, stars have prevailed,” said Sergei Nayakshin of the University of Leicester, the co-author of a study due to appear in the Monthly Notices of the Royal Astronomical Society. “It appears that star formation is much more tenacious than we previously believed.”

Mystery remains
Astronomers still need to figure out how the process works. Many had expected that the high-speed movement of material near the black hole would prevent star formation.

Either the immense disc of gas that orbits the black hole helps fuel the creation of new stars — what scientists call the disk model — or it may serve as a nursery for a cluster of lost stars in a process called the cluster migration model.

In the disc model, the gravity within the dense disc of gas that surrounds Sagittarius A* offsets the gravitational tug from the black hole and allows stars to form. As high-speed jets of radiation blast out of the black hole, they send a supersonic shockwave through the gas cloud, which compresses and heats the gas. The shockwave also ionizes the gas by taking away some of its electrons.

Chandra image of the supermassive black hole in the middle of our galaxy, a.k.a. Sagittarius A* or Sgr A*.

After the shock has passed, the cloud contracts and the ions recombine, creating radiation and transporting energy out of the cloud. The cooling causes the cloud to contract even more, and when a ball of gas becomes dense enough, it can collapse to form a star.

In this model, food for the black hole is being stolen to create stars, countering conventional black hole models that the accretion disc is the engine that feeds the black hole.

“Moreover, these stars did the stealing so efficiently that they became uncommonly heavy,” Nayakshin said during a teleconference Thursday. “An average star here is at least 10 times more massive than an average star elsewhere in the galaxy.”

No migration
In the migration model, the stars actually form in a cluster far away from the black hole and migrate in to form a ring around it. This scenario predicts there should be about a million low-mass stars accompanying the massive stars.

“The only problem observed here is that this star cluster should be very heavy, roughly a million star masses,” said Nayakshin. “What we’ve found there is that you can’t hide more than 10,000 young low-mass stars there instead of a million, so clearly the cluster model is ruled out. So we are quite confident that stars did form in the disc.”

Settling on the disc model presents its own set of problems, though. In most star clusters, low-mass stars comprise about 90 percent of the cluster’s mass, with thousands of young, light stars surrounding a few rare massive stars. Since the cluster around Sagittarius A* is lacking in low-mass stars, scientists will now have to rethink theories of star cluster formation.

“You see, what’s unusual here is the high-mass stars normally are very rare, sort of like whales in the ocean, whereas low-mass stars are sort of like tuna in the ocean — there are much more of them. What is interesting here is that you definitely see the whales, because they are very bright, but you don’t see as much tuna as expected,” Nayakshin told Space.com. “So, whatever theory you may want to build to explain the formation mechanism of these stars, you have to do it in a way that would produce much fewer low-mass stars per one high-mass star.”